1. Field of the Invention
The present invention relates to an exposure apparatus and device manufacturing method.
2. Description of the Related Art
In the lithography process to manufacture a semiconductor element, liquid crystal display device, and the like, a fine-line-width circuit pattern formed on a reticle or the like is projected onto a photosensitive substrate such as a wafer coated with a resist layer, and exposed using a high-resolution projection exposure apparatus.
In the projection exposure process, a volatile substance or the like produced from a photosensitive resin used to form the resist layer applied to the wafer may attach to the surface of an optical member in a projection optical system to contaminate the surface of the optical member. Therefore, in the projection exposure apparatus, operation of wiping the surface of the optical member of the projection optical system, that is, the cleaning operation, by the operator is indispensable. Regarding the timing of performing the cleaning operation, the cleaning operation is performed when the operator visually confirms the contaminated state, or at a predetermined constant periodic timing.
In recent years, the semiconductor integrated circuits shrink more and more in feature size, and the line width uniformity of a transferred pattern is required of a semiconductor exposure apparatus. As a factor that degrades the line width uniformity, the influence of flare occurring in the projection exposure optical system has become no longer negligible. The flare occurring in the projection exposure optical system makes the illuminance distribution within the exposure screen nonuniform to cause a nonuniformity in the line width of an optical image within the screen. The projection exposure optical system refers to a combination of an illumination optical system and projection optical system. The flare of the projection exposure optical system occurs when the reflectance of the surface of the optical element increases due to contamination or the like on the optical element surface.
Conventionally, the flare of the projection exposure optical system is estimated from the exposure result to a wafer coated with a resist. For example, the flare amount occurring in the entire projection exposure optical system is predicted from a change in line width of a pattern image exposed by the semiconductor exposure apparatus, or a change in amount of a resist film outside the exposure region which remains after exposure is performed a plurality of number of shots by changing the exposure amount.
Japanese Patent Laid-Open No. 11-283903 proposes an automatic measurement means which measures contamination of the surface of an optical element in a projection optical system which constitutes a projection exposure optical system. The technique described in Japanese Patent Laid-Open No. 11-283903 discloses an automatic measurement means for measuring the reflectance of the surface of an optical element which is the closest to a photosensitive substrate, and an automatic photo-cleaning means for removing contamination on the surface of the optical element which is the closest to the photosensitive substrate.
According to the technique described in Japanese Patent Laid-Open No. 11-283903, a measuring light beam is obliquely incident on the surface of the optical element on the photosensitive substrate side from outside the projection optical system without passing through the projection optical system, and the reflected light amount of the obliquely incident measuring light beam is measured. Therefore, it is difficult to measure the reflectance on the inner surface of an inner optical element which constitutes the projection optical system. In this manner, with the prior art, it is difficult to measure an increase in flare of each of the optical elements that constitute the projection optical system.
According to the technique described in Japanese Patent Laid-Open No. 11-283903, a light source is additionally provided between the projection optical system and photosensitive substrate to implement the automatic measurement means. This complicates the arrangement of the exposure apparatus.